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Cleaning the Undo Stack

NSUndoManager does not retain the targets of undo operations, for several reasons. Foremost is that the client—the object performing undo operations—typically owns the NSUndoManager; thus for the NSUndoManager to retain the target would create cycles. The NSUndoManager does contain references to the targets of undo operations, however, which it uses to send undo messages when undo is performed. If a target object has been deallocated, and an undo message is sent to it, errors result.

To remedy this, the client must take care to clear undo operations for targets that are being deallocated. This typically occurs in one of three ways:

• The client is the exclusive owner of the NSUndoManager and the target of all undo operations. In this case the client can simply release the NSUndoManager in its dealloc method or deconstructor.

• The client shares the NSUndoManager with other clients. To handle this the client should send removeAllActionsWithTarget: (argument of self) to the NSUndoManager before releasing it in its dealloc method.

• The client registers objects other than itself for undo operations. Here either the client must watch for the other objects being deallocated in order to send removeAllActionsWithTarget:, or the other objects must do so themselves when deallocated (which requires that they have a reference to the NSUndoManager). This is likely to be needed with invocation-based undo.

In a more general sense, it sometimes makes sense to clear all undo and redo operations. Some applications might want to do this when saving a document, for example. To this end, NSUndoManager defines the removeAllActions method, which clears both stacks.

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